1. Field of the Invention
The present invention relates to an optical head for optically recording or reproducing information on or from a plurality of types of information recording media, a method for manufacturing this optical head and an optical disc device provided with this optical head.
2. Description of the Background Art
In recent years, with the practical use of blue-violet semiconductor lasers, Blu-ray discs (hereinafter, “BDs”), which are high-density and high-capacity optical information recording media (hereinafter, also referred to as “optical discs”) having the same size as CDs (compact discs) and DVDs (digital versatile discs), have been put to practical use. This BD is an optical disc which has a protective substrate thickness of about 0.1 mm and on or from which information is recorded or reproduced using a blue-violet laser light source having a wavelength of about 400 nm and an objective lens with a numerical aperture (NA) increased up to 0.85.
CDs, DVDs and BDs differ in protective substrate thicknesses. Accordingly, there has been proposed a compatible optical head for recording or reproducing information by focusing laser lights of different wavelengths on information recording layers of the optical discs having different protective substrate thicknesses using one objective lens.
Here is described an optical head for recording or reproducing information, for example, on or from a CD having a protective substrate thickness of 1.2 mm and a DVD having a protective substrate thickness of 0.6 mm by focusing an infrared laser light and a red laser light using one objective lens. FIG. 29 is a schematic diagram of a conventional optical head.
In FIG. 29, an optical head 130 is provided with a red laser light source 101, an infrared laser light source 111, diffraction gratings 102, 112, a dichroic prism 113, a beam splitter 103, a collimator lens 104, a mirror 105, an objective lens 106, an anomorphic lens 107, an objective lens actuator 109 and a light receiving element 110. Further, a DVD 70 is an optical disc having a protective substrate thickness of 0.6 mm and a CD 80 is an optical disc having a protective substrate thickness of 1.2 mm.
First of all, the operation of the optical head 130 in the case of recording or reproducing information on or from the DVD 70 is described. A red laser light having a wavelength of 655 nm and emitted from the red laser light source 101 is separated into a main beam as a 0th-order light and a sub beam as ±1st-order lights by the diffraction grating 102. The main beam and the sub beam pass through the dichroic prism 113 and the beam splitter 103, are converted into substantially parallel lights by the collimator lens 104 and are bent by being reflected in a direction toward the DVD 70 by the mirror 105. The two beams reflected by the mirror 105 are focused as light spots on an information recording layer of the DVD 70 through a protective substrate by the objective lens 106. The main beam and sub beam reflected by the information recording layer of the DVD 70 pass through the objective lens 106 again and are reflected by the mirror 105 to pass through the collimator lens 104. The two beams having passed through the collimator lens 104 are reflected by the beam splitter 103 and pass through the anomorphic lens 107 to be introduced to the light receiving element 110. The light receiving element 110 generates an information signal and a servo signal using the received main beam and sub beam.
Next, the operation of the optical head 130 in the case of recording or reproducing information on or from the CD 80 is described. An infrared laser light having a wavelength of 785 nm and emitted from the infrared laser light source 111 is separated into a main beam as a 0th-order light and a sub beam as ±1st-order lights by the diffraction grating 112. The main beam and the sub beam are reflected by the dichroic prism 113, passes through the beam splitter 103, are converted into substantially parallel lights by the collimator lens 104 and are bent by being reflected in a direction toward the CD 80 by the mirror 105. The two beams reflected by the mirror 105 are focused as light spots on an information recording layer of the CD 80 through a protective substrate by the objective lens 106. The main beam and sub beam reflected by the information recording layer of the CD 80 pass through the objective lens 106 again and are reflected by the mirror 105 to pass through the collimator lens 104. The two beams having passed through the collimator lens 104 are reflected by the beam splitter 103 and pass through the anomorphic lens 107 to be introduced to the light receiving element 110. The light receiving element 110 generates an information signal and a servo signal using the received main beam and sub beam.
A focus error signal used to record or reproduce information on or from the DVD 70 and the CD 80 is detected using an astigmatism method or the like by the anomorphic lens 107. A tracking error signal is detected using a so-called three-beam method or differential push-pull method (DPP method) or the like using the main beam and sub beam generated by the diffraction grating 102 or the diffraction grating 112.
The objective lens 106 has a diffraction structure for focusing a red laser light for recording or reproducing information on or from the DVD 70 and an infrared laser light for recording or reproducing information on or from the CD 80 as very small light spots utilizing a wavelength difference.
Accordingly, by using such an optical head 130, laser lights having different wavelengths can be focused on different types of optical discs using one objective lens 106 to record or reproduce information.
In this way, in the optical head for focusing laser lights having different wavelengths on different types of optical discs using one objective lens, a coma aberration amount generated by the tilt of the objective lens, i.e. so-called lens tilt differs depending on the wavelength of a laser light. Thus, if a skew adjustment (tilt adjustment) of the objective lens 106 is, for example, performed to minimize the coma aberration for the DVD 70, the coma aberration for the CD 80 may not be minimized in some cases.
To deal with such cases, a construction for performing a skew adjustment of an objective lens to attain a tilt angle optimal for recording or reproduction on or from an optical disc (e.g. DVD) of a specified specification and changing the tilt of the objective lens (lens tilt) by a predetermined angle with respect to the optimal tilt angle in the case of using an optical disc of another specification (e.g. CD) is disclosed, for example, in an optical disc device of Japanese Unexamined Patent Publication No. 2007-66442.
FIG. 30 is a diagram showing emission points of a monolithic two-wavelength laser light source for emitting a red laser light and an infrared laser light. An emission point 131a for emitting a red laser light and an emission point 131b for emitting an infrared laser light in a monolithic two-wavelength laser light source 131 are arranged at a specified distance δ from each other. The monolithic two-wavelength laser light source 131 has such a structure that, for example, a GaAs substrate is arranged on a submount 82, and the red and infrared laser lights are emitted from one laser chip. The two-wavelength laser light source can be of the hybrid type including two laser chips arranged side by side besides being of the monolithic type as shown in FIG. 30. In either type, the emission point for red laser light and the emission point for infrared laser light cannot be generally matched.
Accordingly, if the optical axis of an objective lens and that of the red laser light emitted from the two-wavelength laser light source are matched, the optical axis of the objective lens and that of the infrared laser light emitted from the two-wavelength laser light source cannot be matched. In other words, in this case, the infrared laser light is incident off-axis on the objective lens and coma aberration occurs in the objective lens not satisfying a sine condition.
Although the construction of the optical disc device for correcting coma aberration remaining after the skew adjustment of the objective lens is disclosed in Japanese Unexamined Patent Publication No. 2007-66442, coma aberration generated due to the above displacement of the emission points is not mentioned at all.